What Will Happen if I Consume Too Much Calcium?

It is tempting for people to suppose that if a little of something is good for them, then a lot of it must be better. Unfortunately this is not always or even usually the case; and I first realized that people are inclined to make this mistake when, as a student many years ago, I was shown a baby who was bright orange; it was suffering from a condition known as carotenemia. The parents, having heard that carrots were healthy, concluded that only carrots were healthy, and fed their baby accordingly.

A study from Sweden, recently published in the British Medical Journal, examines the important question of whether calcium supplements are good for middle-aged and old women. The question is important because millions of women around the world take such supplements – 60 percent of American middle-aged and old women, for example. There is no one quite like the Swedes for carrying out such epidemiological studies because the medical records of their population are by far the most comprehensive in the world: creepily so, one is sometimes inclined to think.

What the Swedish researchers found was that the graph of the relationship between calcium intake and death rates was a U-shaped curve. People with a low consumption of calcium had a higher mortality than those with a moderate consumption, but so did people with a high consumption.

The sample of women was not small, and in the period of study 11,944 of the 70,259 women studied had died. Those with a high dietary consumption of calcium alone had an increased death rate of 1.4 times for all causes of mortality, 1.49 times for cardiovascular mortality, and 2.14 times for ischaemic heart disease (heart attacks) compared with those whose who consumption of calcium was associated with the lowest mortality, that is to say a moderate consumption.

It was worse for those with a high dietary consumption of calcium who in addition took calcium supplements, either in the form of multivitamins or calcium tablets. Their all-cause mortality was 2.57 times that of those with a moderate consumption of calcium. According to the authors, “among women with a high dietary intake of calcium, the addition of calcium increased the risk of death in a dose dependent fashion.” This is evidence that the relationship was not just that of statistical association, but of causation.

Interestingly, the women who took calcium supplements, whether as multivitamins or as calcium tablets, were likely to live healthier lifestyles than those who did not: to exercise more, eat more healthily, etc. In other words, they were probably more inclined to do things for the sake of their health, even those things that in the event turn out not to be so good for the health. As Hegel said in another connection, the Owl of Minerva takes wing by night.

There was something I did not quite understand, or at least that surprised me, in the figures given by the authors of this paper. I quote:

11,944 women died; of these women, the underlying cause of death was cardiovascular disease in 3862 ischaemic heart disease in 1932, and stroke in 1100.

Now cardiovascular disease encompasses, among other things, ischaemic heart disease and stroke, that is to say 3032 of the 3862 cardiovascular deaths, leaving 830 others. I think this large latter number will come as a surprise to any doctor.

Nevertheless, the paper is potentially important, especially if it is replicated elsewhere (for you should never put your faith in one study alone, especially when it depends upon the manipulation of complex statistics), for it suggests that the benefit of calcium supplementation is not only the medical equivalent of an urban myth, but a dangerous one at that.